Field of the Disclosure
This disclosure relates to the semiconductor laser technology and in particular to a high power laser diode module characterized by increased stability.
Prior Art
Rapid evolution of semiconductor laser technology has made the adoption of high-power laser diodes more readily affordable. The continuous pursue for higher lasing power calls for better thermal management capability in the packaging design to facilitate the controlled operation. As these laser diodes generate large amount of heat fluxes that can adversely affect their performances and reliability, a thermally-effective packaging solution is required to promptly remove the excessive heat generated in the laser diode to its surroundings.
For high-power applications, one needs to consider not only the thermal challenges, but also the mechanical integrity of the joints in the module. These poses significant packaging challenges as further discussed factors complicate the effort to create an ideal packaging design.
Referring to FIG. 1 illustrating a typical laser diode package, the heat generated in the laser diode is transferred to the ambient environment by attaching the diode onto a sub-mount. The laser diode must be attached to the package optimally to ensure an efficient heat transfer through the thermal interface. A thin thermal bonding interface, such as a soft solder, creates an effective heat dissipation channel through the die attachment process. Based on the illustration, to improve the thermal design of the laser diode package, it is desirable, among others, bringing the heat source, such as the diode laser, to the heat sink as close as possible, making the soft solder interface as thin as possible, increasing the thermal conductivity of the material, and providing an intimate thermal contact between the laser diode and the heat sink.
Thermal bonding interfaces or solders are utilized in every part of a laser diode assembly due to their electrical interconnect, mechanical support and heat dissipation capabilities. These solders can be commonly categorized into two types: hard solder and soft solder. In general, the solder material must satisfy the following requirements:
Have the desired processing temperature to support high temperature operation
Reduce thermally induced stresses due to the mismatch of thermal expansion between the laser diode and heat sink
Exhibit no/low deformation during its long-term operation
Exhibit low electrical resistivity of a sub-mount-die solder to reduce Joules heating at high injection current.
Soft solder, commonly containing a large percentage of lead, tin and indium, has very low yield strength and incurs plastic deformation under stresses. Their capability to deform plastically helps to relieve a stress developed in the bonded structure. However, this makes a soft solder subject to thermal fatigue and creep rupture, causing long-term reliability problems.
Traditionally, as shown in FIG. 1, soft indium/Sn/Bi etc., —based solder is used to bond a heatsink to ceramic sub-mount. The thermal expansion coefficient (“TEC”) of the heat sink made, for example, from copper, does not match that one of the sub-mount. The repeat on-off cycle in laser operations, further referred to as a thermal cycle, can cause mechanical stresses due to different TECs of respective sink and sub-mount which leads to the soft layer material cracking/shearing, etc., and/or indium migration. These defects initially lead to elevated temperatures of the diode. In time the layer of soft solder is destroyed which results in decoupling of the submount from the heatsink and eventual destruction of the laser diode from overheating. This is particularly relevant to high power lasers, as they have a large contacting surface between the die and heatsink and high temperature difference between on and off cycles.
Hard solder, on the other hand, has very high yield strength and thus incurs elastic rather than plastic deformation under stresses. Accordingly, it has good thermal conductivity and is free from thermal fatigue and creep movement phenomena. Regrettably, melting temperatures of known hard solders may be too high and may compromise the integrity of the submount to heat the sink attachment.
A need therefore exists for improved thermo-cycle-resistant laser diode packaging resulting in a high power stable laser diode module.